1. Field of the Invention
The first present invention relates to a method of scanning an information recording medium and a scanning apparatus for an information recording medium that scans for defects in resin layers of an information recording medium which has a plurality of recording layers.
Also, the second present invention relates to a method of scanning and a scanning apparatus that scan a resin layer, which has been formed on a substrate used to manufacture an information recording medium, for defects.
2. Description of the Related Art
As one example of a scanning apparatus according to the first invention, Japanese Laid-Open Patent Publication No. 2002-286653 discloses an automatic defect scanning apparatus that scans for defects in layers such as a reflective layer, a recording layer, and a resin layer (protective layer) of an optical recording medium. As shown in FIGS. 2 and 3 of the publication, this automatic defect scanning apparatus includes a lamp unit, two CCD sensors, a pickup, various kinds of circuits, and the like. In this automatic defect scanning apparatus, the lamp unit emits parallel light to irradiate the disc (an optical recording medium). When, for example, a defect is present in the reflective layer of the disc, a first of the CCD sensors detects light that has passed through the disc and outputs a detection signal. The second CCD sensor detects light that has been emitted by the lamp unit and reflected by the disc and outputs another detection signal. When a defect is present in the recording layer or the resin layer, a shadow produced in the reflected light by the defect is detected by the second CCD sensor. On the other hand, the pickup emits a laser beam onto the disc, detects light reflected by the disc, and outputs a detection signal. Here, the various circuits specify the lengths of the defects based on the detection signals and when the length of the defect exceeds a set length, the disc is judged to be defective. In this automatic defect scanning apparatus, since both scanning with emitted parallel light and scanning with an emitted laser beam are carried out, it is possible to judge with relatively high accuracy whether a disc is a defective disc with a defect in the reflective layer, the recording layer, the resin layer, or the like.
As one example of a scanning apparatus that scans an information recording medium using one example of a method of scanning according to the second invention, a protective film application judging apparatus (hereinafter simply “judging apparatus”) is disclosed by Japanese Laid-Open Patent Publication No. 2000-28748. As shown in FIG. 11 of this publication, the judging apparatus includes a laser source that emits laser light (scanning light) used to irradiate an optical disc (information recording medium) being scanned, a PSD that receives light that has been reflected by the optical disc and outputs a current signal in accordance with the state of the received light, a rotation control unit that rotates the optical disc mounted on a base, and a microcomputer (see FIG. 12 of the publication) that detects defects based on the output signal of the PSD. On the other hand, the optical disc being scanned has films such as a dielectric film, a magnetic film, and a reflective film formed on a substrate (base) made of polycarbonate or the like, with a lubricating protective film (hereinafter, simply “protective film”) for protecting the reflective film also being formed. This protective film is formed by first applying a UV curing resin in a ring at a central part of the optical disc (substrate), rotating the optical disc at high speed to spread out the UV curing resin uniformly across substantially the entire area of the optical disc using centrifugal force, and then exposing the resin to UV rays.
With this method, when, for example, the applied amount of UV curing resin is insufficient during the formation of the protective film, the UV curing resin cannot spread out to cover the entire area of the optical disc, and regions where the resin is not applied (i.e., defects) appear on the optical disc. Since proper recording and reproduction become problematic for the optical disc 9 when such defects are present, it is necessary to scan for the presence of such defects when manufacturing optical discs. More specifically, while an optical disc for which the formation of the protective film has been completed is being rotated by the rotation control unit, the optical disc is irradiated with laser light emitted by the laser source from the surface on which the protective film has been formed. When doing so, if the protective film of the optical disc is not flat (that is, when there are defects such as unapplied regions or irregularities in the application), the laser light emitted toward the protective film is reflected irregularly at the positions of the defects. This means that the amount of reflected light incident on the PSD falls, or there is a change in the incident position of the reflected light incident on the PSD. Accordingly, based on the reflected light incident on the PSD, the microcomputer judges that a defect is present when the amount of reflected light falls or when the incident position changes. At this time, the positions (detection sectors) where defects are detected are displayed on a display device. In this way, the scanning of the disc is completed.
By investigating the automatic defect scanning apparatus described above, the present inventor discovered the following problem. That is, with the automatic defect scanning apparatus described above, although it is possible to scan for defects in the reflective layer, the recording layer, and the resin layer, there are problems, for example, when scanning an information recording medium on which are laminated a plurality of recording layers that are separated by a plurality of resin layers. More specifically, to improve the manufacturing yield for products, it is preferable not only to identify products with defects but also, for example in a formation process for the resin layer, to correct formation conditions, such as the temperature of a resin material and rotational velocity used during spin coating, based on the scanning results so as to reduce the occurrence of defects. In this case, for an information recording medium with a multilayer structure, the respective resin layers normally have different thicknesses, so that the formation conditions for the respective resin layers are set separately. Accordingly, to correct the respective formation conditions, it is necessary to specify the resin layer in which a defect has occurred. However, with a conventional automatic defect scanning apparatus, it is only possible to scan for the simple presence of defects and not possible to specify the resin layer in which a defect is present, so that there is the problem that it is difficult to reduce the occurrence of defects in the resin layers.
Also, by investigating the method of scanning according to the judging apparatus described above, the present inventor discovered the following problem. That is, in the method of scanning used by the judging apparatus, when an optical disc is irradiated with laser light from the surface on which the protective film has been formed, it is determined that defects such as unapplied regions or irregularly applied parts are present in the protective film when the amount of light drops or the incident position changes for the reflected light that is incident on the PSD. To increase the recording density, current optical discs have increasingly high densities for data recording tracks (i.e., increasingly narrow pitches for data recording tracks). This means that it is necessary to reduce the beam spot diameter of the laser beam used during the recording and reproduction of data. Accordingly, even if there are only minute irregularities (defects) in the lubricating protective film (a hard coat layer or the like), the laser beam emitted onto the optical disc will be irregularly reflected at the defective parts, which prevents data from being recorded and reproduced properly. This means that during the manufacturing of optical discs, it is now necessary to scan for extremely fine defects that were not especially problematic for older optical discs. However, there is the problem that by using a conventional method of scanning where an optical disc is irradiated with laser light from the surface on which the protective film is formed, it is difficult to detect extremely fine defects that can cause problems for modern optical discs.